This invention relates to implantable medical devices such as defibrillators and automatic implantable defibrillators (AIDs), and their various components. More particularly, it relates to an implantable medical device including a flat capacitor with case liner configured to optimize an overall size and shape of the device.
Implantable medical devices (IMDs) for therapeutic stimulation of the heart are well known in the art. Examples of various forms of IMDs and their respective functions include: a programmable demand pacemaker disclosed in U.S. Pat. No. 4,253,466 issued to Hartlaub et al. to deliver electrical energy, typically ranging in magnitude between about 5 and about 25 micro Joules, to the heart to initiate the depolarization of cardiac tissue to treat the heart by providing pacemaker spike in the absence of naturally occurring spontaneous cardiac depolarizations; an automatic implantable defibrillator (AID), such as those described in U.S. Pat. No. Re. 27,757 to Mirowski et al. and U.S. Pat. No. 4,030,509 to Heilman et al., deliver a nonsynchronous high-voltage energy pulse (about 40 Joules) to the heart to interrupt ventricular fibrillation through widely spaced electrodes located outside of the heart, thus mimicking transthoracic defibrillation; a pacemaker/cardioverter/defibrillator (PCD) disclosed in U.S. Pat. No. 4,375,817 to Engle et al., to detect the onset and progression of tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycardia or fibrillation; an external synchronized cardioverter, such as that described in xe2x80x9cClinical Application of Cardioversionxe2x80x9d in Cardiovascular Clinics, 1970, Vol. 2, pp. 239-260 by Douglas P. Zipes, provides cardioversion shocks synchronized with ventricular depolarization to ensure that the cardioverting energy is not delivered during the vulnerable T-wave portion of the cardiac cycle; an implantable cardioverter, such as those disclosed in U.S. Pat. No. 4,384,585 to Douglas P. Zipes and in U.S. Pat. No. 3,738,370 to Charms, detect the intrinsic depolarizations of cardiac tissue and pulse generator circuitry delivers moderate energy level stimuli (in the range of about 0.1 to about 10 Joules) to the heart synchronously with the detected cardiac activity.
An IMD consists generally of a sealed housing maintaining a capacitor(s), an electronics module(s) and an energy source. The electronics module normally includes a circuit board maintaining a variety of electrical components designed, for example, to perform sensing and monitoring functions or routines, as well as to accumulate data related to IMD operation. The electronics module is electrically connected to the capacitor and the power source such that amongst other functions, the electronics module causes the power source to charge and recharge the capacitor. To satisfy power and safety requirements, the power source typically consists of two series-connected batteries. So as to optimize volumetric efficiency, the batteries are typically formed to assume a cube-like shape. For example, a well accepted IMD configuration includes two, three-volt cube-like batteries connected in series.
Typically, the electrical energy required to power an implantable cardiac pacemaker is supplied by a low voltage, low current drain, long-lived power source such as a lithium iodine pacemaker battery of the type manufactured by Wilson Greatbatch, Ltd. or Medtronic, Inc. While the energy density of such power sources is typically relatively high, they are generally not capable of being rapidly and repeatedly discharged at high current drains in the manner required to directly cardiovert the heart with cardioversion energies in the range of 0.1 to 10 Joules. Moreover, the nominal voltage at which such batteries operate is generally too low for cardioversion applications. Higher energy density battery systems are known which can be more rapidly or more often discharged, such as lithium thionyl chloride power sources. Neither of the foregoing battery types, however, may have the capacity or the voltage required to provide an impulse of the required magnitude on a repeatable basis to the heart following the onset of tachyarrhythmia.
Generally speaking, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high-energy storage capacitor. Charging of the high-energy capacitor is accomplished by inducing a voltage in the primary winding of a transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding which is applied to the high-energy capacitor to charge it. The repeated interruption of the supply current charges the high-energy capacitor to a desired level over time.
Energy, volume, thickness and mass are critical features in the design of IMDs. IMDs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams. One of the components important to optimization of those features is the high voltage capacitor used to store the energy required for defibrillation. Such capacitor a typically deliver energy in the range of about 25 to 40 Joules.
It is desirable to reduce the volume, thickness and mass of such capacitors and devices without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the device. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the device, or balanced addition of new components, thereby adding functionality to the device. It is also desirable to provide such devices at low cost while retaining the highest level of performance.
Most conventional IMDs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, December 1989, Year Book Medical Publishers, Chicago, and U.S. Pat. No. 4,254,775 for xe2x80x9cImplantable Defibrillator and Package Therefor.xe2x80x9d The electrodes in such capacitors are typically spirally wound to form a coiled electrode assembly. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle anode foils from fracturing during coiling. The anode, cathode and separator are typically wound around such a paper core. The core limits both the thinness and volume of the IMDs in which they are placed. The cylindrical shape of commercial photoflash capacitors also limits the volumetric packaging efficiency and thickness of an IMD made using same.
Recently developed flat aluminum electrolytic capacitors have overcome some disadvantages inherent in commercial cylindrical capacitors. For example, U.S. Pat. No. 5,131,388 to Pless et al. discloses a relatively volumetrically efficient flat capacitor having a plurality of planar layers arranged in a stack. Each layer contains an anode layer, a cathode layer and means for separating the anode layers and cathode layers (such as paper). The anode layers and the cathode layers are electrically connected in parallel.
A segment of today""s IMD market employs flat capacitors to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. Examples of such flat capacitors are described in the ""388 patent to Pless et al. for xe2x80x9cImplantable Cardiac Defibrillator with Improved Capacitors,xe2x80x9d and in U.S. Pat. No. 5,522,851 to Fayram for xe2x80x9cCapacitor for an Implantable Cardiac Defibrillators.xe2x80x9d Additionally, flat capacitors are described in a paper entitled xe2x80x9cHigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d by P. Lunsmann and D. MacFarlane presented at the 16th Capacitor and Resistor Technology Symposium.
Numerous efforts have been made to improve upon the size, shape and performance characteristics of the various IMD components. For example, implementation of a flat capacitor configuration has greatly improved IMD performance as well as reducing and improving the size and shape of the IMD housing. Similarly, advancements in electrical component technology has greatly reduced size requirements associated with the electronics module, along with facilitating use of a lower voltage power source (e.g., three-volt versus six-volt). Along these same lines, enhancements in materials and construction techniques used for IMD batteries have resulted in the reduction of sizes and costs.
A flat aluminum electrolytic capacitor stack is built as descried in earlier disclosures. Commercially available cylindrical capacitors as well as flat aluminum electrolytic capacitors described in prior art patents such as ""851 Pless et al. typically employ the use of a metal housing, such as aluminum or an aluminum alloy. Electrical insulation from the cathode elements is not employed. Electrical isolation from the anode elements is typically employed by using separator elements (e.g., a paper layer) that overhang the edges of the electrode plates, thereby separating the anode electrode elements from the metal case. The case is either directly connected to the cathode elements through a welded joint or through contact with the electrolyte.
Reducing the size of aluminum electrolytic capacitors, while at the same time increasing the energy storage capacity per unit volume or energy density requires the minimization of non-energy storage elements. One way to reduce volume in flat aluminum electrolytic capacitors without reducing the amount of energy storage is to reduce or eliminate the length of paper that overhangs the edges of a flat capacitor stack. However, as this separator overhang is decreased the potential for contact between the edges of anode plates and the case wall increases. Close proximity may also result in arcing between the edges of the anode plates and the case wall at sufficient voltages. Elimination of separator overhang may also result in arcing between the edges of the anode plates and cathode plates.
A further problem with flat aluminum electrolytic capacitors that use a stacked plate type design is the relative movement of anode, cathode, and separator layers which may result in direct anode to cathode shorting paths or greater susceptibility to anode to case arcing. Use of alignment elements has been employed in the design of some conventional flat capacitors; however, these elements usually add inert volume on the order of 0.5 to 1.0 cc, while reducing the energy storage surface area of the anode/cathode. Yet another problem with conventional flat aluminum electrolytic capacitors is the incidental introduction of outer paper layers into the case-to-cover joint. This joint seal is conventionally formed by compression or weld. The presence of foreign material in the joint, such as separation layer paper, may result in a failed joint seal due to a blown weld or insufficient/leaky crimp rendering the capacitor assembly not fit for use.
To avoid the shortcomings of the above-discussed techniques and for other reasons presented in the Description of the Preferred Embodiments, a need exists for an IMD incorporating a capacitor having superior space-volumetric efficiencies to thereby advance the preferred objectives for continuing IMD size reduction, longer electrical IMD lifespan, higher reliability, lower cost and/or increased functionality.
One aspect of the present invention provides an implantable medical device including a housing, a capacitor assembly, an electronics module and an energy source, such as a substantially flat battery. The capacitor assembly is disposed within the housing. The electronics module is electrically connected to the capacitor assembly and is disposed within the housing. The energy source is electrically connected to the electronics module.
In one preferred embodiment of the present invention an insulative barrier such as a case liner is utilized in the capacitor assembly between the electrode stack and the conductive capacitor case element. Prior to being inserted into the case, a case liner element or elements is placed around the perimeter of the capacitor assembly electrode stack. The case liner provides the necessary insulation and isolation that allows for further reduction in separator overhang even to the level of no overhang.
In one embodiment, the capacitor assembly electrode stack is first inserted into the case and a case liner slid into place around the capacitor assembly electrode stack. In another embodiment, the liner is first placed inside of the case and the capacitor assembly electrode stack is either inserted into the case liner or the electrode stack may be constructed directly into the liner. The capacitor assembly case liner aligns and immobilizes the capacitor assembly electrode stack. The capacitor assembly liner is especially effective in reducing shifting of the electrode stack upon insertion into the case. The liner design may be realized in many different preferred embodiments, including two-piece construction. The capacitor assembly case liner may separate into two pieces along one of any of the three dimensions. For example, the capacitor assembly case liner may consist of top and bottom portions, left and right portions, or front and back portions. Alternatively, the capacitor assembly case liner is formed as a box having one hinged side. In another embodiment, the capacitor assembly case liner is a single piece construction folded around the capacitor assembly electrode stack. The liner may be constructed of a variety of insulative materials, and by a variety of methods. A preferred method uses a thermo-forming technique. Alternatively, the liner may be machined, injection molded, or thin film coated onto the capacitor assembly case.